Method for preparing pentamethylene silicone polymers



Patent a mam-non ron mar mo PENTAME ENE smrcom POLS Joseph M. Hersh,Ponw City, Gkla, assimor to Continental Oil Company, Ponca City, Okla" acorporation of Delaware a No Drawing. Application June 18, "1945,

Serial No. 600,239

3 Claims. (Cl. 260-4482) This invention relates as indicated to organoFormula 4 cycle-silicon condensation products and relates not only tosuch products as new materials per se, but also to improved methods for.the manufacture of such materials. 5

- It is a further and more particular object of a c c this invention toprovide novel compositions of i -z- \H matter including the aboveidentified products as R R essential t t t, v 1 In Formulas 3 and 4above, Z is an element of Other objects or the invention will appear asthe roup consisting of oxy en. Sul r. and n the description proceeds. S

To the accomplishment of the foregoing and n m l s 1 and 2 a v X resentsa h related d d invention t comprlses h drolyzable and/or condensableradical such as features hereinafter fully described and particuny ofthe following: larly pointed out in the claims, the following de-Hamgens scription setting forth in detail certain illustrativeembodiments of the invention, these being Chlorine sulfhydl'ylindicative, however, of but a few-of the various Iodine Amino ways inwhich the p inciple oi the inventionmay Fluorine Immo be employed. miOxide The monomeric cycle-silicon compounds to Hydmxyl Sulfide which thepresent invention relates'have the fol- In each of"th for g ing f rm lmay be 1wmg characterizing structures: hydrogen or any organic radicleand R. may be any organic radical such as the following rep- Fomul 1 iresen-tative organic radicals which will be found H present in thepreferred compounds of this invention.

(a) Alkyl radicals (normal or branched chain) R s1=x r a a e. 8.. i I QPropyl Octyl 3 Amy] The isomeric amyls Isobutyl The isomeric hexylsFormula 2 Hexyl The isomeric heptyls R n a n 5 (b) lradicals.e.g.:

c Phenyl Naphthyl 11 8i08i \R p nyl Anthracyl c it c (c) Aralkylradicals, e. 5.: 7 4o B a n Phenyl-propylfl Phenyl-hexyl Di hen 1 butlP01 1 -In their condensed form, these cyclo-silicones A prefellredyclassif this iv g f are f 110 in m irical g'g g i by the o w 8 e p those inwhich an aryl radical is substituted in $5 an aliphatic carbon chainsuch as 1-3, 1-4 and Formula 3 preferably 1-5 diarylalkanes from whichhighly H active di-halides maybeformed.

(ai Alk'aryi radicals, e. g.: 5 Methyl-phenyl Dodecyl-naphthyl /lPropyl-phenyl Wax-phenyl \H Amyl-phenyl Wax-diphenyl Cetyl-phenyl Arepresentative formula. for the dimeric con- In certain cases, the arylgroup may form, in

densed cyclo-siliconesisasiollows: p t. the cy l -slliwn s r ct s ch aswhen.

two silicon valences are to an a yl radical preferably in the 1-4position.

(e) Cyclo non-benzenoid radicals, e. g.:

Di-cyclohexyl Cyclopentyl Cyclohexyl Phenyl-c "clopentyl CyclobutylWax-cyclohexyl Methyl-cyclopentyl It will be observed that in certaincases the carbocyclic radical, in whole or in part, forms thecyclo-silicon structure.

(1) Heterocyclic radicals, e. g.:

Furyl Phenyl-fury Pyridyl Octadecyl-furyl Thiophenyl It will be observedthat in certain instances,

the cyclic radical, in part, permits of a dihalide' derivative which isreactive to link silicon in a cyclo-silicon compound forming a bicyclicradical, including a heterocyclic silicon radical.

In each of the foregoing formulas, B. may be any of the radicalsidentified under R above, or

alternatively, either or both of B may be inorganic and selected fromthe following list which characterize the preferred compounds of myinvention:

Hydrogen, Halogen,

Chlorine Iodine Bromine Fluorine Nitrogen, Phosphorus, Sulfur, eithersimple orsubstituted.

In each of the Formulas 1 to 4 above, Si denotes a silicon atom.

In each of Formulas 1 to 4 above, 11 is an integer equal to at least 2.

It will be observed that the unit structure of Formula 3 above is basedupon the disilico-ether unit For a fuller understanding of the nature ofmy improved products and the novel methods by which the same may beprepared, reference may be had to the following examples: I

EXAMPLE I.SYNTHESIS or CYCLO-SILICON HALIDES These compounds having thefollowing representative structure, viz:

H Cl

may be prepared by a procedure represented by 4 anhydrous ether diluentuntil all the halide has been added. The reaction may require warming atthe start or the addition of a reaction promoter such as a crystal ofiodine or a drop or two of an active alkyl iodide such as ethyl iodide.when the reaction has started, the reactor should be strongly cooled tomaintain only a slight, regular reflux of ether. Cooling is thengradually withdrawn and heating reinstituted to maintain an even etherreflux for three hours. The Grignard compound separates as an oilybottom layer. Then with rapid stirring and sharp cooling, such as withan ice-brine bath around the reactor, one molal part of silicontetrachloride in ether solution is added slowly. Finely dividedinsoluble magnesium salts separate from the solution and the reaction isessentially complete when the silicon halide has been added and wellstirred. The organo-silic'on product is essentially cycle-pentamethylenesilicon dichloride. However, some monochloride and some trichloride arepresent. For many purposes the mixed cyclo-silicon halides, essentiallythe dichloride, may be hydrolyzed directly. I prefer to separate anessentially pure cyclo-silicon dihalide, in this case, in order toprepare a preferred primary cyclo-silicone by hydrolysis.

The reaction product is transferred under as near anhydrous conditionsas possible to a distillation Vessel, decanting the liquid from theseparated solids. The solids are washed with anhydrous ether which isadded to the product, By distillation at atmospheric pressure the etheris removed; then the product is fractionated to remove essentially purecyclo-pentamethylene silicon dichloride, a clear liquid boiling at165-175 C. at atmospheric pressure or at lower temperatures underreduced pressure. Fractionation is preferably at reduced pressure.

' Instead of using 1,5-dibromopentane in the preparation of thecyclo-sllicon compound, any

bromo phenethyl chloride, m-di-iodotoluene, 1-

chloro-4-bromobenzene, 1,4-dichloronaphthalene, etc. or any of thehalogenated R or R organic radicals as shown above.

The preferred halide reactant should be in a carbon chain of at leastthree carbon atoms and preferably five carbon atoms such that thecyclosilicon structure formed has a maximum stability inherent informing and being part of a 4 to 6 or more membered ring.

EXAMPLE II.--HYDROLYZED CYCLO-SILICON HALIDES When the cyclo-siliconhalides as per Example I above are hydrolyzed in accordance with theimproved process of my invention, a product having the followingrepresentative structure produced:

Formula 6 H HE where n is not more than 5. The product is a lowmolecular weight mobile liquid and not a high molecular weight hardresin such as has 1 volumes of a low boiling inert solvent such as moreaccurate control condensation by chemical ccndensingagents makes thisthe preferred means for carrying out the desired reaction. It

- will generally be found preferable to employ a benzene and graduallyadded to a well stirred bath of ice-water or ice-brine with carefulcontrol so that hydrolysis is effected with a minimum of condensation.The product is extracted with benzene (the solvent used above) andconcentrated in the benzene solution or recovered from the benzene bydistilling oil the solvent. The removal of benzene by vacuumdistillation effects both a cooling of the reactants and amorecompletecontrolled lwdrolysis by the concominitant removal ofvvolatile hydrogen halides.

EXAMPLE III.-Connnnsnn PRIMARY Crcno- Snrcorms The primarycyclo-silicones prepared by the improved hydrolysis method of myinvention, as represented by Example II above, may be condensed to formproducts having a, structure like that given in Formula 6 aboveexcepting that n in this case is from 5 to- 50. A representative andpreferred procedure in the preparation of such products which may bereferred to for convenience as the intermediate polymer of thecyclo-silicone, having a molecular weight of from 600 to 6000, is asfollows: The primary cyclopentamethylene silicones from Example II,preferably diluted with an equal volume of a non-condensable.hydrocarbon solvent such as a straight run parafllnic naphtha is treatedwith one volume of concentrated sulfuric acid and warmed gently withstirring until an apparently uniform solution is obtained. The reactionmixture is allowed to remain in contact for a requisite time to obtainthe desired degree of condensation. For a lower molecular weight polymerwhere n is from 5 to 50 about one-half hour of contact is suflicient.The product is then diluted with water to release the condensing agent,washed with dilute caustic solution when an acidic condensing agent hasbeen used, separated and dried.

The oily product produced by the procedure just described may beseparated from the hydrocarbon solvent but in most cases it will befound preferable to utilize the hydrocarbon solution of thecyclo-silicone for the purposes hereinafter more specifically explainedfor which these end products have been found to be particularly useful.

In the procedure outlined above, concentrated sulfuric acid has beenreferred to as the condensing agent. The strength of the condensingagent may be varied with the activity of the condensing agent and theease of effecting condensation. With the lower molecular weightcycle-silicon -ydihalides or the related primary silicones, condensationtakes place quite readily so that a more dilute condensing agent and ashorter period of time may be employed for the controlled polymers. Withhigher molecular weight cyclosilicones, halides, or ether halides thecondensation is more readily controlled and the use of concentratedagents is preferred. Heat alone may be used to effect the condensation,but the avoidance of thermal dissociation, the greater homogeneity ofthe condensed product and the nols or aminophenols.

chemical condensing agent such as the following E804, HSOaCl, HSOaFHaPOs, 114F201, HPO:

P205, P235. PC]: PCls ZnClz, CoClz, MnCl:

FeCla, AlCla, AlBra HF, BF:

etc.. and for conciseness these are hereinafter referred to asacid-acting chemical condensing agents.

The length of time during which the primary cyclo-silicones are exposedto the condensing action of any of the chemical agents listed above willdetermine the degree of condensation which takesplace. Thus bycontrolling the degree of condensation of a condensable cyclo-silicone,as readily measured by molecular weight or viscosity change, I produce aselected compound having an optimum polymeric structure for a specificuse. Upon attaining the desired degree of condensation, the condensablemass is freed of the condensingagent (by water washing if it is achemical agent) and inhibited from further undesirable condensation bystorage at low temperature and in solution in a non-condensinghydrocarbon medium, or preferably by the action of condensationinhibitors which are of the class of organic compounds miscible with thecondensed silicone or thiosilicone having a labile group such as thealcohols, esters, amines, phe- Ethyl alcohol, isopropy'l amine, certainethanol amines, eth'yl silicate, ethyl borat phenol p. aminophenol.N-tert butyl p-aminophenol, phenyl silicate, ethyl acetate and .the likeare efiective in this function to a detion with the cyclo-silicone, suchthat its removal from the condensed silicone need not be undertaken andin fact may advantageously be avoided. Thus a small amount of ethylalcohol or ethyl silicate, for example, may be tolerated in an oilysystem in which the condensed silicone is used to control frothing inthe processing of an oily-organic mixture as disclosed in my copendingapplication, Serial No. 564,705. Similarly, small amounts of the moreliophilic condensation inhibitors may be tolerated in the lubricantcomposition incorporating condensed organo silicon compounds asdisclosed in copending applications Serial Nos. 527,136, 527,137,564,703 filed November 22, 1944,-now abandoned, and 519,188. I have thusprepared stable, homogeneous and versatile cyclo-silicones of from 500to 10,000 molecular weight.

Exsmrm IV.Ar.rr:nmrrvr Memo or SYN- THESIZING CYcLo-SrucoN HALIDESInstead of using the synthesis method of Example I above, I may preparedisilicon halides,

which are highly eilective in producing the dimore I I in which siliconatoms are linked through an intermediate linking element such as oxygenby using a material such'as silicon oxychloride to producehalo-cyclo-silico-ethers from which condensed poly-dimensional polymersmay be produced by controlled'hydrolysis and condensation. An example ofsuch procedure is as follows:

Silicon oxychloride (one molal part), magnesium molal parts) andanhydrous ether are placed together in a reactor and 1,4 dichlorobutaneis added in small quantities. Some time elapses before any noticeablechange occurs and it may be necessary to warm gently or add a crystal ofiodine to initiate the reaction; but when the reaction has once started,it normally proceeds vigorously with a considerable development of heat.The reactor is therefore strongly cooled to avoid boiling the ether,using an outside ice or ice-brine solution. The rest of the1,4-dichl0ro-butane is then added slowly either directly or in ethersolution (two molal parts of dichlorobutane) While the contents of thereactor are rapidly stirred. During this process, the

magnesium is gradually changed to magnesium .salts and the latterseparate as a solid, heavy,

coarsely crystalline powder. As soon as the whole of the butyldichloride has been added, the etheral solution of the silicon compoundmay be separated by filtering and extracting in the usual way or bygradually heating the reactor and distilling off the ether and then thecrude halide. The crude mixture of cyclo-silicon halides thus preparedmay be used-directly in the subsequent hydrolysis. The combined productmay be bydrolyzed rapidly in ice water or in a hydrolyzing bath heldsubstantially at 0 C. or lower and washed to remove residual HCI. Theprimary di-cyclo-silicol-ether thus prepared has two hy droxyl groupsper molecule and may be further condensed on heating or by chemicalagents to produce the preferred condensed di-cyclo-silicone. The primarydi-cyclo-butyl silicol-ether is a practically colorless limpid liquid ina primary state of condensation. When the oily product is kept overconcentrated sulfuric acid, it very gradually condenses giving off waterto form more highly condensed cyclo-butyl silicone. By this gradualcondensation method, the reaction can be stopped when the optimum degreeof condensation has been effected, then by stabilizing the siliconesolution, for example, by dissolving the silicone in an inertnon-condensing hydro carbon, or by adding an inhibitor of condensationas shown above. The preferred additive may be kept for a considerableperiod of time.

EXAMPLE V.-DIRECT PREPARATION or POLYMERIC CYCLO-SILICONES The directpreparation of a polymeric cyclosilicone may be carried out veryeffectively using active metals such as magnesium, zinc, aluminum,sodium, lithium, potassium or alloys such as lithium-sodium,sodium-potassium, and the like to effect the coupling. It is highlydesirable, for example to bring about the'reaction as described above(Example IV) with metallic sodium molal parts) in the form of moltendispersed sodium particles in a dry inert solvent medium such as tolueneor xylene. -Under these higher temperature conditions permissible withsilicon oxyhalide, an organic dihalide, such-as paradichlorobenzene,reacts instantaneously to produce the desired chlorodicyclophenyl-silicol ether, which may then be carefully hydrolyzed inthe same reaction vessel with controlled cooling or warming to give thedesired primary silicol-ether in a low state of polymerization forfurther condensation in the presence of condensing agents as shownabove.

EXAMPLE VIP-PREPARATION or CYCLO-BILIOOK ESTERS A particularly effectiveand preferred means of converting and then condensing thehalo-cyclosilicon compounds is by the use of alcohols to eifect analcoholysis of the halogenated compounds as prepared in the aboveexamples. For example, the chloro-dicyclobutyl-silico-ether of ExampleIV is treated under controlled conditions of cooling with analcohol or ametal alcoholate such as sodium methylate and/or ethyl alcohol (3 molalparts) to effect the removal of halogen from silicon and replacing suchhalogen with an ester group 0R. This ester type of compound is not asreadily hydrolyzed by atmospheric moisture or water as the halide andthe cyclo-silicon compound is stabilized for protracted storage orhandling. At the selected time the ester type compounds, where the estergroup is from a simple alcohol such as methyl, ethyl, phenyl, andsoforth, may then be treated with a condensing agent such as. constantboiling hydriotic acid (127 C. boiling point) to produce a controlledcondensate. On boiling the silicon compound with H1 in inert solutionssuch as benzene for a modest period of time, from 15 minutes to an houror longer, the degree of condensation can.

be controlled to produce the desired polymeric form. By this meanscertain active cyclo-silicon compounds, particularly thecycle-silicones, have been prepared in optimum degree of activity, forexample where .n, the integer indicating the degree of molecularcondensation as shown above in the formulas, is from 5 to 50 andpreferably from 30 to 40, such that the condensed compound has amolecular weight approximately 4000. This compound has extremely activeanti-foaming properties such that one part of the above material in from100,000 to 1,000,000 parts or more of oil practically eliminates thefoaming tendency of such oil. On further treating such halogencontaining esterifled or partially condensed silicon compound with acondensing agent such as BF'TI, HI, AlCla, and the like, the more highlycomplex cyclo-silicones have been produced.

The various materials referred to above will be found to be preferablefor use as compared with a the silicones and halogenated silicones ofthe prior art as represented by U. S. Patents Nos. 2,258,218; 2,258,219;2,258,220; 2,258,221; and 2,258,222, which relate to the preparation ofresinous materials, either aliphatic or aromatic in which the siliconestructure has no cyclo-silicon nucleus. For example, the methylsilicones disclosed in the above-identified patents are resinousmaterials having from 1 to 2 methyl groups per silicon atom and theethyl silicones have from /2 to 1 /2 ethyl groups per silicon atom.These 1 compounds are especially resinous in character as described insaid patents. The compounds contemplated by the present invention are ofa heterocyclic structure in which silicon is bound in an organo-carbonring. This structure thus simulates an aryl-silicane of extremely highstability, but is .diiferent in that the silicon is part of the ring.Instead'of havin alkyl or aryl groups attached to silicon by a singlevalence bond, the system of compounds to which this invention relateshas an organic chain linked to silicon by two valences. "Thus molecularcomplexity is built up in the cycle-organo constitueats-o! silicon to anextent not shown in the prior art. This characteristic of molecularcomplexity afiects the physical and chemical properties of thecompositions so prepared, especially their solubility in hydrocarbontype solvents and oils, their surface, active eiiectin oil orhydrocarbon solutions such as influences their anti-frothing oranti-foaming properties, and especially their chemical stability. Thisseries of compounds has these outstanding and other differences from theknown organo-silicones of the art.

By the linking of a carbon. chain of at ,least three carbon atoms tosilicon by two valcnces in a cyclic structure of at least four unitmembers,

the molecular structure 'is stabilized and made more complex throughinternal linkages. For example, in the dihydrocarbon silicols of theprior in the cyclic structure of this invention. the added cycliccomplexity provides a two-dimensional polymer whereas the prior artnon-cyclic silicol provides only a linear polymer.

Further, by the use of a silicon oxy-halide such as SiaOClc asthe'basicsilicon reactant, it is possible to prepare a dimeric silico ethercontaining a cyclo-silicon structure on each silicon atom and havingthen a residual halogen for hydrolydihalides used in the presentinvention may be coupied with the silicon reagentby means of the activecoupling metals such as lithium, sodium, potassium and potassium-sodiumalloy, and the like, in inert solvents such as'benzene, toluene xylene,and the like, at elevated temperatures under which conditions the activemetals are molten and finely dispersed and, therefore, in'a most activestate. By this means the cycle-silicon compounds are advantageouslyformed at a relatively low cost.

The compounds produced in accordance with my invention have a widevariety of uses. The lower polymeric'forms are particularly useful forpurposes such as: I (a) anti-foam agent in blended heavy duty oils inconcentrations of from .00001 per cent to0.1 percent, whichconcentration eifectively elimin ates foaming in such lubricants underconditions of mechanical agitation. A

(b) anti-frothing agents-in oily-organicpro cessing systems in'concentrations of from 0.0001- per cent to 0.1 per cent, whichconcentrations are effective in eliminating or controlling frothing: in

ization of fatty acids and organic acids, rendering 40 of fats a'ndthelike, dehydration anddeaerati'on sis and controlled condensation. Inthis type of compound there is provided a new composition of matterhaving particular utility in fields such as a light, my liquid havingparticular antifrothing action in chemical processes or as a viscous,oily liquid having particular use as an anti-foam agent in lubricatingoil compositions,

. or as a more viscous product having higher One of theparticularadvantages of my invention is in the method of manufacture.Thus, while methyl or ethyl si'icon halides and silicones of the priorart must necessarily be prepared by meansof tlie Grignard-type compoundsusing magnesium or zinc in anhydrous ether, the correspondingcycle-silicon compounds of the present invention, especially when usingsilicon oxyhalide. can be prepared under higher temperature conditionsand under the catalytic and coupling effect of the alkali metals bywhich means the halogen atoms of both the silicon reagent and theorganic reagent are coupled and e'iminated forming the organosiliconcomplex for further hydrolysis and condensation.

While methyl and ethyl halides have low boiling points such that areaction involving 'these of greases, halogenation, hydrogenation,dehal'ogenation and dehydrogenation of oils, fats, and waxes, etc.

In certain lubricating compositions concentrations of the condensedcyclo-silicones of from 0.1 per cent to 10 percent or more may be usedas viscosity improving agents and stabilizing agents. In this range ofconcentrations, I have found that the halogen-bearing or halogenatedsilicones have excellent extreme pressure and load carrying propertiessuch that aconiposition containing 0.1 per cent or more of the haiogenbearing silicones show high Timken load test. The cyclo-' siliconcompounds are particularly e fiective in this regard when the halogen(preferably chlorine) atom or atoms are attached to carbon atoms in thealpha or primal position relative to the silicone atom: -For example inthe generic formula of the cyclo-silicones l I R/ SiZ-- /i i thehalogenated species have the highest order of efiectiveness as extremepressure agents when.

the groups designated as 0" above are halogen substituted. The secondaryposition for halogen substitution. but still of outstanding importanceis on a carbon atom beta to the silicon atom,

whether this be in the cyclo-silicon ring or on a side chain such as anR group attached to "6. The halogen atoms so substituted have acontrolled activity which apparently gives them the 11 degree ofinstability suited to overcoming metal seizure under the conditions ofboundary lubrication.

when additional quantities of halogens are introduced in thecyclo-silicon structure, these polyhalogenated silicones form anextremely viscous, tough thermotropic composition having high resistanceto ignition and to attack by insects or fungi. These compositions serveas excellent fire-proofing, fungus and insect-proofing components ofimpregnating or coating baths for fabrics, ropes, organic plastics, woodand other susceptible construction materials.

The higher polymeric forms of the compounds of my invention are usefulfor a wide variety of purposes such as for electrical insulation,coating, impregnating, etc.

The organo cyclo-sllicon condensation products of the present invention,and particularly those in which n is from to 50, are especially adaptedfor use in lubricating compositions comprising a mineral lubricating oilto which there has been added a material which has the property ofincreasing the foaming tendencies of the oil. The materials of thepresent invention will be found particularly useful when combined withproducts, such as those disclosed in U. S. Patents Nos. 2,139,766;2,197,833, 2,197,834; 2,197,835; 2,228,661; and 2,228,671, the materialsof such patents being generically classifiable as phenates and arecustomarily employed in lubricating compositions for the purpose ofincreasingthe detergency of the oil. Another class of detergents withwhich the compounds of the present invention may be advantageouslyemployed in combination are the sulphonates, that is, the metallic saltsof sulphonic acids and particularly the oil-soluble metallic salts ofpetroleum sulphonic acids.

For an additional list of usages to which the compounds of the presentapplication are particularly applicable, reference may be had to thecopending applications Serial No. 564,705, filed November 22, 1944;Serial No. 538,204, filed May 31, 1944, now forfeited, Serial No.564,703, filed November 22, 1944, now abandoned; Serial No. 519,188,filed January 21, 1944; Serial No. 527,137, filed March 18, 1944; SerialNo. 527,136, filed March 18, 1944; in each of which applications I ameither a sole or Joint inventor.

Other modes of applying the principle of the invention may be employed,change being made as regards the details described, provided thefeatures stated in any of the following claims or the equivalent of suchbe employed.

1, therefore, particularly point out and distinctly claim as myinvention:

1. The method of producing an oily polymer of the following structure:

where n is from 5 to 50 which comprises hydrolyzing a cyclo-siliconhalide in an ice bath to produce a primary cycle-silicone having theabove structure but in which n is not more than 5, recovering suchprimary cycle-silicone and dissolving it in an inert solvent. andcontacting such solution for a period of about one-half hour with amineral acid condensing agent.

, 3. The method of producing an oily polymer of the following structure:

where n is from 5 to 50 which comprises hydrolyzing a cyclo-siliconhalide in an ice bath to produce a primary cyclo-silicone having theabove structure but in which n is not more than 5, diluting such primarycycle-silicone in a noncondensable hydrocarbon solvent, and contactingsuch solution for a period of about one-half hour with an acid-actingchemical condensing agent to produce said oily polymer dispersed in aninert solvent.

JOSEPH M. HERSH.

REFERENCES, crrEn The following references are of record in the file ofthis patent:

STATES PATENTS 7 Number Name Date 2,258,220 Rochow Oct. 7, 19412,352,974 Rochow July 4, 1944 2,371,050 Hyde Mar. 6, 1945 2,371,068Rochow Mar. 6, 1945 2,381,002 Patnode Aug. 7, 1945 OTHER REFERENCESHart, Jahresbericht uber die Fortschitte der Chemie, 1889, pages1943-1945.

Bygden, BerichteDeut. Chem. GeselL," vol. 48, Pages 1238-1242 (1915).

Hyde, "Jour. Am. Chem. Soc.-, vol. 63, pages 1194-1196.

